1. Field of the Invention
The present invention relates generally to an image forming apparatus employing a screen type photosensitive medium (hereinafter referred to as screen) and more particularly relates to an improvement of air flow line system provided within such image forming apparatus.
2. Description of the Prior Art
One example of the screen pertinent to the present invention is a multilayer screen comprising a photoconductive material and an electrically conductive material and, if necessary, further an insulating material, and having a great number of fine openings through which ion stream or the like may be passed.
Such type of screen is used in an image forming process in the following manner.
Initially, a primary electrostatic latent image is formed on the screen by subjecting it to charging and image-wise exposure. Thereafter, a secondary electrostatic latent image is formed on an electrically chargeable member by controlling the ion stream passing through the openings of the screen making use of the electric field formed at the openings owing to the first latent image. The second latent image thus formed on the electrically chargeable member is developed or visualized at the next step for further use of it.
The above described image forming method is known. For example, such image forming process is disclosed in Japanese Patent Application Publication No. 5063/1973 (its counterparts are U.S. Pat. No. 3,645,614 and DOLS No. 1,910,392), U.S. Pat. Nos. 3,647,291, 3,680,954 and 3,986,871.
An important problem involved in this image forming process employing a screen is that the electric resistance of the screen surface is gradually decreased. This is caused by corona discharge made for the screen at the primary latent image forming step. When a corona discharge is applied to the screen, dust and other contaminants in air and various chemical compounds formed by ozone produced during corona discharging such as nitrogen dioxide (NO.sub.2), nitrogen monoxide (NO) and ammonium nitrate (NH.sub.4 NO.sub.3) are deposited on the screen by the corona discharge. These contaminants adhered onto the screen gradually change in quality and/or absorb moisture from the atmosphere and thereby the electric resistance of the screen surface is gradually decreased with time.
Such decrease of the surface electric resistance prevents the screen from being charged up to a sufficient potential. Therefore, the screen having these contaminants deposited thereon can produce only such a primary electrostatic latent image having a reduced potential or lowered electrostatic contrast. As a result, the quality of image finally produced becomes deteriorated.
As a solution to the problem, it may be considered that the contaminants are to be removed mechanically. But, since the screen is extremely thin and soft, it is difficult to apply to the screen directly a cleaning means as usually used in an electrophotographic apparatus.
An effective approach to solve the problem is to form an air stream flowing toward the side of corona discharger passing through the screen at the time of charging during which the contaminants are most apt to deposit onto the screen. The flow of air toward the corona discharger prevents dust and other contaminants from depositing on the screen.
Another possibility to prevent the screen from being contaminated with dust and the like is to keep the air in the vicinity of the corona discharger substantially free from dust and the like.
These conceptions of use of air flow have already been realized in an image forming apparatus employing a screen. For example, such image forming apparatus in which air flow is used to prevent the contamination of the screen is disclosed in Our U.S. application Ser. No. 729,692, U.S. Pat. Nos. 3,936,184 by Yujiro Ando et al. and 4,040,731 by Masaji Nishikawa. These prior inventions were all made to solve the above described problem.
To prevent the contamination of a screen with dust, however, it is not always necessary to continuously form an air flow toward the discharger all the time during which the image forming apparatus is in operation. The deposition of dust on the screen must be prevented by flowing air toward the discharger at least only when the latter is in operation.
On the other hand, the image forming apparatus employing a screen often includes such means and part which also require an air flow or are operative with air flow, in addition to the above mentioned primary latent image forming part. Examples of such air using means and part are a sheet separating and conveying mechanism for separating a transfer sheet carrying thereon a secondary latent image from a related drum using a back pressure, a mechanism disposed in the vicinity of the developing means for preventing the scatter of developer using an air flow, a cooling air stream generating mechanism disposed for the fixing means and driving source, and an ozone treating mechanism for decomposing ozone accumulated within the apparatus due to corona discharge.
In all the known image forming apparatus of the type employing a screen, no measure is adopted to fully control the air flow throughout the apparatus even though only an air flow for the screen is taken into consideration. Therefore, air is flown unnecessarily in the apparatus, which results in reduction of operational efficiency of the apparatus and also may cause the problem of noise.